The present invention relates to a method for soft annealing of high carbon steel.
Soft annealing normally takes 12 to 48 hours time and is performed either batchwise or continuously in an oven. The load in the oven is then heated to about 800.degree. C. which takes between 2 and 10 hours, the temperature is maintained for about 2 hours, the temperature is then quickly brought down to about 790.degree. C. and then down to about 690.degree. C. at a rate of about 10.degree. C. per hour.
This procedure is very time consuming, costly and may result in decarburization.
Further, because of different conditions at different locations in the oven, the structure will vary substantially between the objects, and also within one and the same object. A test of a batch of tubes of standard steel, SAE52100, showed that the hardness varied between 170 and 220 HB, depending on where in the oven the respective tube was placed.
When soft annealing a batch of tubes, one tube can be subjected to different conditions over its length, resulting in thermal stresses, and in a considerable distortion at the subsequent hardening.
There is a great need of reducing the costs involved in the soft annealing process for high carbon steel. However, the structure of steel after the soft annealing process is of critical importance for subsequent procedures and for the intended use. Many attempts have been made to develop the soft annealing procedure in different aspects.
According to JP04103715-A (Sumitomo Metals Ind.) high carbon chrornium bearing steel is subjected to spheroidizing treatment; first by heating to 780-820.degree. C. and cooling to below Arlb point at less than 200.degree. C./hr and by heating to Aclb-(Aclb+40).degree. C., cooling to below Arlb at less than 200.degree. C./hr, heating to Aclb-(Aclb+40).degree. C. and cooling to below Arlb at less than 75.degree. C./hr. This publication deals mainly with the structure of the steel, and does not teach how to solve the problems discussed above.